Dominique Francois Jean Arago

Francois Arago was educated first at Perpignan, then at the école Polytechnique in Paris. He became professor of analytical geometry at the école Polytechnique at the age of 23. Later he became director of the Paris Observatory.

Arago was also active in other ways. He served for many years as secretary of the Académie des Sciences. He was active politically for the republican cause. He served several political roles in government in addition to his scientific duties. In the provisional government which took power after the 1848 Revolution he was minister of war and marine. He introduced many reforms while holding this government position. This government established universal manhood suffrage in France.

Arago made early discoveries on the corpuscular theory of light in 1811. Working with Fresnel he discovered that two beams of light polarised in perpendicular directions do not interfere, leading to the transverse theory of light waves.

His theory of light predicted that the velocity of light should decrease as it passes into a denser medium. In 1838 he described a test to compare the velocity of light in air and in water or glass. However difficulties with the experiment meant that Arago was not in a position to try his experiment until 1850. By this time however his sight had become poor, so it was left to others to carry out a refined version of the experiment. Successful results were obtained by Fizeau and Léon Foucault before Arago died.

In 1820 the Danish physicist H C Orsted produced experimental results on electricity and magnetism. Arago carried out further experiments of this type and demonstrated several effects which led Faraday later to explain them as induction.

Working with Biot Arago made measurements of arc length on the Earth which led to the standardisation of the metric system of lengths. He suggested that his student Le Verrier investigate irregularities in Uranus's orbit and, after Neptune was discovered, participated in the argument regarding naming the planet and with Adams regarding priority.